Since Robey U.S. Pat. No. 4,023,506, it has been known that a flat layer of artificial fibers disposed below the ground can be used to enhance the stability of a natural turf located thereabove, by providing enhanced holding power for the root system of the natural grass. Thereafter, U.S. Pat. Nos. 4,044,179 and 4,337,283, issued to Haas, Jr., disclosed a surface comprising an artificial composite having a horizontal backing and upright fibers, with sand filled in to provide lateral support for the fibers. The primary purpose of the inventions disclosed in these Haas patents was to provide a stable, maintenance free surface for a variety of uses, such as a landing area for golf ball driving ranges, or even golf courses.
European patent application EP 0 403 008 A1 discloses a combination of natural grass and artificial fibers which extend upwardly from a horizontally disposed artificial backing to stabilize the natural grass. The roots of the natural grass extend downwardly through the backing, the grass extends upwardly above the fibers, and the artificial fibers are supported laterally by a fill layer, such as sand. Japanese Patent Application No. 59-151838 also discloses a combination surface of natural grass and artificial fibers, with the fibers secured to a decay resistant backing. This application states that the combination surface is particularly suitable for golf course tee boxes.
U.S. Pat. Nos. 5,489,317 and 5,586,408, issued to Bergevin, disclose and claim a structure similar to these prior disclosures, but with the specific feature of the sand fill layer terminating at the same vertical level as the tops of the artificial fibers. This results in an above-ground playing surface which is entirely natural grass, because the fibers do not extend above the sand. As indicated in the Bergevin patents and in the prosecution histories thereof, a relatively thick layer of sand is necessary to fill the fibers to their upper ends, to uniformly hold down the horizontal backing, and also to initiate natural plant growth at a vertical level which is very near to the top ends of the artificial fibers.
Such a substantial sand fill layer atop the horizontal layer and surrounding the artificial fibers forms a tight, uniformly compacted playing surface of natural grass. However, because of the height of the sand, i.e., to the tops of the fibers, the ground surface tends to become slick and hard once the natural turf begins to wear. The reason for this problem requires a discussion of the importance of turf management practices. It is a commonly understood turf management practice to apply periodic layers of "topdressing," usually sand, on a natural turf athletic surface, such as a ball field or golf course putting green or tee. The periodically applied layers of topdressing provide stability and some degree of firmness to the ground, but more importantly they create and help maintain an environment conducive to healthy plant growth and rapid recovery from use.
Most well kept athletic surfaces for sport purposes are maintained in such a way that the grass clippings are collected, rather than simply allowed to disperse on the surface. However, with natural grass plants, particularly under the intense maintenance regimes common for professional and collegiate facilities, even if clippings are collected there is still considerable production and decomposition of plant and root matter which can quickly build up and form a thatch layer. This decomposing plant matter will eventually form a thick organic layer if not managed. Agronomically, this organic layer has severe negative effects on drainage and movement of nutrients and pesticides into the root system. From an athletic playability standpoint, footing, surface speed, wear tolerance, ball roll and bounce, can be adversely affected. The organic layer can result in a condition known in the industry as "black layer," whereby the organic matter becomes anaerobic due to poor drainage and a loss of capacity for gas exchange. This condition can severely limit root growth and overall plant health.
It is well recognized that the periodic application of "topdressing," (i.e., a layer of growth media, usually sand or sand based, with a thickness in the range of about 0.125-0.25 inches), helps to prevent the forming of an organic layer of this type. Topdressing also ameliorates these harmful conditions if such a layer should begin to develop. Topdressing is typically applied 1-3 times per year. The number of applications depends to some extent on the availability of the field. The surface cannot be used immediately after topdressing because topdressing creates a loose layer on the playing surface, which adversely affects footing in the short term. With respect to the turf disclosed in the Bergevin patents, the application of the first layer of topdressing would immediately result in completely burying the upright artificial fibers, so that the fibers would be diminished in their ability to perform the intended purpose of providing lateral stability for the turf. Thus, the owner of the field must choose between the lesser of two evils, i.e., completely burying the stabilizing fibers with the first application of topdressing or risking the possibility that an adverse "black layer" may eventually form at the ground surface. It is applicant's understanding that the written turf management guidelines for the Bergevin turf product specifically emphasize that topdressing should not be applied to the turf because it would negate the beneficial aspects of the product. Stated another way, this product forces the owner of an athletic field to disregard the well known turf maintenance practice of applying topdressing. Thus, the wear of the turf eventually causes slickness, but one of the normal turf management steps for ameliorating this condition is discouraged.
Another commonly recommended and practiced turf management strategy involves aerating a natural turf As with applying topdressing, aerating is periodically performed to decrease the build-up of organic matter on the surface, decrease compaction, keep the surface well drained and to maintain a vigorous natural turf canopy. Typically, aerating is done with the same frequency or in conjunction with topdressing, due to scheduling, although additional aerations without topdressing may provide further agronomic benefits. But again, the frequency depends on field availability and scheduling. Aerating is usually done by inserting and removing into the turf hollow tines which are 0.375-0.75 inches in width. In order to penetrate an entire thatch layer and the uppermost root zone area of the natural turf it is typically necessary to aerate to a depth of three inches or greater.
Unfortunately, with the various prior art natural grass/artificial fiber turfs described above, it can become difficult to aerate down to the desired depth because of the structural density of the horizontal backing which holds the artificial fibers. This is particularly true if aerating is done with hollow aerating tines, which are preferred over solid pointed tines. Also, if aerated too many times, the backing would no longer be able to achieve its stated purpose of holding the fibers. With the product constructed in accordance with the Bergevin patents, the management guidelines recognize this problem, and the turf management manual states that aeration should not be implemented. Thus, while this structure initially provides increased stability for the natural turf surface, the horizontal backing hinders the performance of one of the most basic turf management practices, namely aerating with hollow tines. The athletic field owner is again forced to disregard an advantageous turf management principle.
The horizontal backing can also act as a flow barrier between the layers of growth media located above and below. This can adversely affect drainage or subirrigation of the field. This concern arises particularly with respect to subsurface drainage, irrigation and heating systems, and controls therefor, as disclosed in applicant's PCT Application Nos. PCT/US95/11891 and PCT/US96/02207. In essence, the horizontal backing and/or the organic surface layer can create a "perched water table" above the underlying growth media, a condition which is readily understood by those skilled in the art. By creating a perched water table at the top of the root zone, downward movement of water and nutrients and upward movement of water by capillary action ("wicking") during sub-irrigation and the natural phenomenon of evapo-transpiration are significantly inhibited. In essence, the subsurface materials above the backing are isolated from those below it.
Thus, the relatively recent developments in combining artificial fibers with natural grass plants have produced only limited advances in achieving a stabilized natural turf athletic field, particularly but not exclusively for natural turfs which are constructed of soils high in sand content. The relatively slow market acceptance of these stabilized turfs to date indicates that there is room for improvement, primarily from an agronomic standpoint.
A stabilized natural turf may be initially grown on a sod farm, on top of a plastic layer, until it is ready to be cut and then transported to the ultimate site where it is to be used. Since the transporter will typically pay transportation costs which are directly proportional to the weight of the material being transported, it is desirable to minimize the overall weight of the components which make up the stabilized athletic turf grown at the sod farm. Large layers of sand atop the horizontal backing are counter-productive in this regard.